Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) method and apparatus, in particular to an MRI method and apparatus making use of the Dixon technique.
Description of the Prior Art
MRI is an imaging technology involving biomagnetics and nuclear spin which has advanced rapidly with the development of computer technology, electronic circuit technology and superconductor technology. In MRI, human body tissue is placed in a static magnetic field B0, then an RF pulse of a frequency identical to the precession frequency of hydrogen nuclei is used to excite hydrogen nuclei in the human body tissue, resulting in resonance of hydrogen nuclei and absorption of energy. Once the RF pulse is stopped, the hydrogen nuclei emit radio signals at a specific frequency, releasing the absorbed energy; these are received by a receiver outside the body, and processed in a computer to obtain an image.
In MRI, to achieve better imaging quality, it is often necessary to suppress specific spectrum component signals, such as fat signals, water signals and silicone signals (from breast implants). For instance, in MRI examinations of the abdomen and chest, etc., it is generally necessary to suppress fat signals, in order to make tissues of interest or the focus of infection more prominent in the displayed image. Many fat suppression techniques have already been proposed, e.g. the CHESS (CHEmical Shift Suppression, chemical shift selective) technique, the FatSat (fat saturation) technique, the SPAIR (Spectral Presaturation Attenuated Inversion Recovery) technique, the STIR (short inversion time inversion recovery) technique and the Dixon technique, etc.
In cases where the CHESS technique fails to be effective, the Dixon technique is a widely accepted failsafe method. Compared with the CHESS technique, a water image obtained by the Dixon technique has more residual signals from fat, which present a tricky technical problem in a T2 weighted image (in which the contrast between fat tissue and non-fat tissue is very low), where T2 is the transverse relaxation time.
The Dixon technique in the prior art has the following technical feature:
                              s          ⁡                      (            t            )                          =                              (                                          ρ                w                            +                                                ρ                  f                                ⁢                                                      ∑                                          p                      =                      1                                        p                                    ⁢                                      e                                          j                      ⁢                                                                                          ⁢                      2                      ⁢                                                                                          ⁢                      π                      ⁢                                                                                          ⁢                                              f                        p                                            ⁢                      t                                                                                            )                    ·                      e                          j              ⁢                                                          ⁢              2              ⁢                                                          ⁢              π              ⁢                                                          ⁢              ψ              ⁢                                                          ⁢              t                                                          [        1        ]            
Specifically, formula [1] is the signal model of the Dixon technique: wherein the magnetic resonance image signal s(t) comprises a fat signal ρf and a water signal ρw. At the same time, the spectrum of the magnetic resonance image signal includes P fat signal peaks; due to chemical shift, the various peaks of the fat signal undergo modulation by their own frequency, this frequency reflecting a phase shift caused by inhomogeneity in the local magnetic field.